Exposure to certain man-made and natural environmental agents poses a significant threat to human health. In order to enable us to establish rational policies to deal with this health issue, we need to expand our knowledge about the mechanisms by which toxic and carcinogenic environmental agents compromise human health. Toxicity and cancer caused by the nongenotoxic agent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) and the ubiquitous environmental combustion product benzo[a]pyrene (BaP) appear to be mediated by way of the aromatic hydrocarbon receptor (AHR). Previous work from this and other laboratories has shown that (a) there exist inbred mouse strains having genes for high- and low-affinity AHRs;(b) mice with the high-affinity-AHR phenotype metabolize numerous environmental chemicals, to form reactive intermediates, 15-20 times faster than mice with the low-affinity-AHR phenotype; (c) DNA adduct formation, mutagenesis, oncogene activation, and certain types of cancer occur more frequently in mice with the fast- metabolism phenotype; and (d) approximately one-tenth of the human population has a fast-metabolism phenotype, and in this group the risk of certain types of cancer among smokers appears to be several-fold greater than that in the slow-metabolism group. The laboratory animal data for high vs low-affinity-AHR differences in toxicity and cancer are very convincing, whereas the human AHR-related differences in toxicity and cancer remain equivocal, largely due to the ethical difficulties in carrying out definitive experiments in humans. Here we propose to: (1) determine the nucleotide difference(s) in the human AHR gene responsible for human high and low-affinity-AHR phenotype among members in one 3- generation family; (2) make heterozygous and homozygous disruptions of the Ahr gene in murine embryonic stem (ES) cells; (3) replace the murine disrupted gene with the human low-affinity (AHR) gene in one cell line, and in another cell line with the human high-affinity (AHRA) gene; and (4) generate the two human transgenic mouse lines. These animals will be helpful in examining the role of the human high vs low-affinity AHR in toxicity and cancer-- on a common mouse background where every other gene is identical. This project goes substantially beyond what we and others have already done in this field. These mice will be invaluable for defining the precise role of the human Ah receptor in innumerable studies of toxicity and cancer caused by TCDD, BaP and other environmental chemicals. These mice should become important, valid dose- response models needed for a rational approach to human risk assessment.